An ultrasonic distance sensor is traditionally used as an in-vehicle monitoring system configured to monitor the periphery of a vehicle such as a car or the like to detect an obstacle. The ultrasonic distance sensor is configured: to transmit an ultrasonic sound through a transducer (transceiver); to receive the ultrasonic sound (reflected waves) reflected by an object (an obstacle) at the periphery of a vehicle through the transducer; and to measure the distance to the object based on receiving of the reflected waves.
This sort of in-vehicle monitoring system is used for giving an alarm to a driver of the vehicle when a distance between the vehicle and an obstacle is shorter than a predetermined distance, or for superimposing location information of the obstacle on ambient video captured by a camera. What is needed is to precisely determine the presence of an obstacle and the distance to the obstacle regardless of the distance between the vehicle and the obstacle.
Therefore, Japanese Patent Application Publication Number 2005-9992 discloses a vehicle surrounding monitoring system. This system is configured to repeatedly measure a distance between a vehicle and an obstacle with a predetermined period through a distance sensor to determine the distance to the obstacle and the shape of the obstacle based on continuous measurement results (distances between the vehicle and the obstacle), thereby precisely monitoring the surrounding.
In such in-vehicle monitoring systems, as the distance to an obstacle becomes longer, a signal level of reflected waves received through a distance sensor becomes weak. Accordingly, there is a possibility that false detection of obstacle occurs under the influence of noise.
For example, if the number of measurements for detecting an obstacle is increased, a far obstacle can be precisely detected. However, increasing the number of measurements causes a large time lag from first receiving reflected waves by an obstacle to outputting a detection result. Also, when an obstacle exists near the vehicle or in its traveling direction, there is a possibility that an alarm emitted by detecting an obstacle is delayed. In order to reduce the time lag, if the number of measurements for detecting an obstacle is decreased, there is a possibility that a far obstacle or an obstacle in the opposite direction of a vehicle's traveling direction cannot be detected, or false object detections frequently occur under absence of obstacle.